It's center of optic to center of bore. In some rifles, it's possible to take a caliper reading from the bottom of the barrel to the top of the scope tube, and subtract 1/2 the barrel diameter and 1/2 the scope tube diameter.

Zak Smith

March 21, 2005, 04:31 PM

I just string a ruler or caliper next to my rifle and measure from approx the center of the barrel up to approx the center of the scope. On an iron sighted rifle, you can just use the front sight height over the bore center.

A difference in sight height of 'X' will cause a difference in computed vs. actual trajectory of approx X at twice the zero distance.

-z

Morlock

March 21, 2005, 10:39 PM

thanks for the responses, but unfortunately i do not have a flat top receiver handy. I guess i'm more interested in the distance from bore centerline to the top of the rail center channel on a mil spec upper.